1. Field of the Invention
The present invention generally relates to voltage regulation, and more particularly, to a method and apparatus for continuously regulating an output voltage of the charge pump using a capacitor divider network.
2. Description of the Related Art
A non-volatile memory powered from a single low voltage power supply, V.sub.CC, typically employs a multi-phase voltage or charge pump to boost V.sub.CC to a high voltage VPP required for programming and erase operations. Further details for providing a multi-phase charge pump can be found in U.S. Pat. No. 5,263,000 by Van Buskirk entitled "Drain Power Supply " and U.S. Pat. No. 5,511,026 by Cleveland entitled "Boosted And Regulated Gate Power Supply With Reference Tracking For Multi-Density And Low-Voltage Supply Memories ", both incorporated by reference herein. The boosted voltage is regulated to provide stable programming and erase voltages.
FIG. 1 shows a voltage output of charge pump 110 of a non-volatile memory regulated by regulator 120 to generate a constant output voltage VPP. FIG. 2 illustrates a circuit diagram of a typical prior art regulator 120. The supply voltage V.sub.CC powering both charge pump 110 and regulator 120 is, for example, 3 V and the target output voltage of charge pump 110, V.sub.target, is, for example, 9.5 V (it should be noted that all voltage values used herein are approximate values).
Accordingly, if capacitors C1 and C2 are metal capacitors, the voltage at node A is 0 V when the Enable (EN) input is at a logic LOW level and when the initial voltage of VPP is 3 V.
However, once an erase operation, for example, is initiated, the EN input transitions to a logic HIGH voltage level. This turns on P-MOS transistor T.sub.1, and the voltage at node A subsequently increases as charge pump 110 charges up the voltage at nodes B and D to VPP. The voltage at node C remains at 0 V until the voltage at node A exceeds a fixed reference voltage V.sub.ref. The target output voltage of charge pump 110 is at all times proportional to a ratio of the values of capacitors C1 and C2. The following equation expresses this relationship: ##EQU1##
For certain values of capacitors C1 and C2, the voltage at node A is 1.3 V if the target voltage is 9.5 V. However, if charge pump 110 charges VPP beyond the target voltage of 9.5 V, the voltage at node A rises above 1.3 V, and the output of comparator 210 (node C) will transition to a logic HIGH voltage level (since V.sub.ref is fixed at, for example, 1.3 V). This turns on transistor T2 and transistor T2 discharges VPP back down to the target voltage of 9.5 V.
While this feedback mechanism can be effective in regulating VPP to the target voltage, prior art regulator 120 is only effective for regulating relatively short output voltage pulses of charge pump 110 (e.g., up to 2-3 msec). This is because capacitors C1 and C2, like all capacitors, leak charge over time. Consequently, if the output voltage pulses of charge pump 110 were, for example, 20 msec long, the steady state voltage at node A progressively dips below 1.3 V (under normal circumstances, the voltage at node A is 1.3 V when V.sub.PP is charged up to the target voltage of 9.5 V). In response, prior art regulator 120 compensates for this change in the voltage of node A by increasing the voltage at node B. In other words, regulator 120 tries to "pull " the voltage at node A back up to 1.3 V by increasing the voltage at node B. The only way regulator 120 can increase the voltage at node B is to increase the voltage at node D, or VPP.
Thus, for long output voltage pulses of charge pump 110, regulator 120 must increase VPP beyond the target voltage of 9.5 V to maintain the voltage of node A at 1.3 V. Regulator 120 may have to increase VPP to, for example, 11 V in order to maintain 1.3 V at node A. This result is undesirable because regulator 120 should regulate the voltage output of charge pump 110 at the target voltage (e.g., 9.5V) and not at any other voltage. Accordingly, there is a need for a regulator comprising a capacitor divider network that is capable of continuously regulating the output of charge pump 110 at some target voltage. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.